Polypropylene-based resins are lightweight and have excellent recyclability and, hence, there is an increasing demand for application thereof to automotive components. Specifically, polypropylene resin compositions obtained by incorporating an elastomer such as an ethylene/propylene copolymer or an ethylene/butene copolymer and an inorganic filler such as talc into a crystalline polypropylene resin are in use as the polypropylene-based resins. It has been proposed to improve moldability, mechanical properties, appearance, etc. by suitably selecting a crystalline polypropylene resin, an elastomer, and an inorganic filler in accordance with purposes.
Recently, there is a desire for a polypropylene-based resin from which molded articles having a down gauging can be molded in a shorter molding time, from the standpoint of more efficiently producing automotive components reduced in weight. However, such molded articles have a problem in that appearance defects such as flow marks (the pattern of tiger stripes appearing in molding surface) are prone to occur. In particular, in components designed to have an uncoated portion exposed outward such as a bumper, there has been a trouble that such flow marks impair the design attractiveness of the merchandise.
Techniques for improving such flow marks are disclosed, for example, in patent documents 1 to 4.
Patent document 1, which is to provide a polypropylene-based resin composition that is less apt to develop flow marks or gels and attains an excellent appearance, discloses a polypropylene-based resin composition obtained by blending, in a specific proportion, a polypropylene-based resin (A) having a propylene homopolymer portion having an intrinsic viscosity of 1.3 dL/g or less and a propylene/ethylene random copolymer portion having an intrinsic viscosity of 3.0 dL/g or less with a propylene/ethylene block copolymer (B) having a propylene/ethylene random copolymer portion having an intrinsic viscosity of 8.0 to 15 dL/g.
Patent document 2 discloses a process for producing a propylene/ethylene block copolymer which includes a propylene polymer having a MFR of 100 to 1,000 g/10 min and in which a propylene/ethylene copolymer accounts for 5 to 10 wt % of the whole weight.
Patent document 3 discloses a polypropylene-based resin composition which shows satisfactory moldability in injection molding, is excellent in terms of flow mark characteristics, and is suitable for injection-molded articles including automotive exterior components. It has been ascertained that by enlarging the difference in viscosity between the propylene homopolymer and the propylene/ethylene random copolymer, the composition is rendered usable as an additive ingredient which is more effective in improving flow marks.
Patent document 4, which is to provide a polypropylene-based resin composition that attains a satisfactory appearance and has excellent moldability, discloses use of a moldability modifier which includes a propylene/ethylene block copolymer having specific properties. Specifically, document 4 discloses a propylene/ethylene block copolymer in which the propylene homopolymer portion (crystalline component) has a MFR of 500 g/10 min or higher and which as a whole has a MFR of 100 g/10 min or higher and a die swell ratio of 1.2 to 2.5.
These techniques each relate to a propylene/ethylene block copolymer which includes a low-viscosity high-MFR component as a propylene homopolymer portion and a high-viscosity component as a propylene/ethylene random copolymer portion. It is thought that both the improvement in flowability due to a reduction in shear stress attributable to the high-MFR propylene homopolymer portion and the stabilization of flow due to an increase in normal stress attributable to the high-viscosity propylene/ethylene random copolymer portion contribute to the improvement of flow marks in those techniques.
However, it is necessary in those techniques that for controlling the appearance of molded articles by incorporating the propylene/ethylene block copolymer as an additive ingredient, the copolymer should be incorporated in a large amount. There has hence been a problem in that the incorporation thereof undesirably changes some properties of the polypropylene-based resin composition which are desired to remain unchanged.
Propylene/ethylene block copolymers which, when used as additive ingredients, are capable of improving appearance with a small addition amount are disclosed, for example, in patent documents 5 and 6.
Patent document 5 discloses a propylene-based polymer configured of 51 to 75% by weight of propylene/ethylene random copolymer component (A) having an intrinsic viscosity of 5 dL/g or higher and an ethylene content of 8% by weight or higher but less than 20% by weight and 25 to 49% by weight of propylene-based polymer component (B) having an intrinsic viscosity of 1.2 dL/g or less and obtained by polymerizing monomers consisting mainly of propylene.
Patent document 6 discloses a propylene-based block copolymer which is configured of a crystalline propylene polymer portion having an intrinsic viscosity [η]homo of 1.2 dL/g or less and a propylene/ethylene random copolymer portion that has an ethylene content of 30 to 70% by weight and an intrinsic viscosity [η]copoly of 2.5 to 7.0 dL/g and that accounts for 40 to 80% by weight of the whole, and which as a whole has a MFR of 0.1 to 10 g/10 min and a value of [η]copoly/[η]homo of 2.5 to 10.
However, these techniques have had a problem in that incorporation of the propylene/ethylene block copolymer as an additive ingredient results in an excessive decrease in the flowability of the polypropylene-based resin composition.